Despite numerous advances over the years, there exists a continuing need for improved liquid purification. Many areas of the world, for example, have insufficient fresh water for drinking or agricultural uses, and even where plentiful supplies of fresh water exist, the water is often polluted with chemical or biological contaminants, metal ions and the like. There is also a continuing need for commercial purification of other liquids such as industrial chemicals and food juices. U.S. Pat. No. 4,759,850, for example, discusses the use of reverse osmosis for removing alcohols from hydrocarbons in the additional presence of ethers, and U.S. Pat. No. 4,959,237 discusses the use of reverse osmosis for orange juice.
Many of these needs have been addressed by reverse osmosis, which is the removal of contaminants from a liquid by passing the liquid through a membrane under pressure. As used herein, the term "membrane" refers to the functional membrane unit, which may include one or more semipermeable layers and one or more support layers. Depending on the fineness of the membrane employed, reverse osmosis can remove particles varying in size from the macromolecular to the microscopic, and modern reverse osmosis units are capable of removing particles, bacteria, spores, viruses and even ions such as Cl.sup.- or Ca.sup.++.
Despite these successes, there are several problems associated with large scale reverse osmosis (RO), including excessive fouling of the membranes and high costs associated with producing the required pressure across the membranes. These two problems are interrelated in that most or all of the known RO units require flushing of the membranes during operation with a relatively large amount of unpurified liquid relative to the amount purified liquid produced. This ratio, often 3:1 or even 5:1, means that if one simply pumps unpurified liquid up to a storage tank and then allows it to flow down through an RO membrane, the amount of purified liquid produced may not justify the costs of pumping so much unpurified liquid.
There have been numerous attempts over the years to overcome these problems. U.S. Pat. No. 5,229,005 to Fok et al, for example, describes lowering a vessel from the side of boat deep into the ocean. The vessel is equipped with an RO membrane on one of its surfaces, and at a depth of about 2250 feet, the ambient pressure is sufficient to force fresh water through the membrane and into the vessel. When the vessel is thus filled with fresh water, it is raised back to the ship and emptied. To increase operating efficiency, the inventor suggests alternately lowering and emptying two such vessels. While the claimed method can be functional, non-continuous nature of the process renders it largely inadequate to supply fresh water on a commercial scale.
Another attempt at overcoming these problems was discussed in U.S. Pat. No. 4,512,886 to Hicks et al. There, an RO module is placed in the ocean at a depth at which the ambient pressure is insufficient to operate the membrane, but at which the ambient pressure combined with additional pressure provided by a pump is sufficient to operate the membrane. Pressurized water is therefore pumped through the RO module utilizing energy from waves overhead, with fresh water coming out one end of the module and "waste brine" being eliminated from the other end. Unfortunately, the mechanism is limited to localities having considerable wave action, and in any event is relatively costly to install and operate.
Still another attempt at overcoming these problems was discussed in U.S. Pat. No. 3,456,802 to Cole et al. In that patent, several RO cells were submerged at a sufficient depth in the ocean, and salt water was fed to the cells through a pipe from the surface. Fresh water output of the cells was then pumped back up to the surface, while the flush water was released from the RO cells directly into the ocean. By this mechanism Cole et al. claimed to increase membrane life by pre-filtering the salt water applied against the membranes, and by increasing the flushing rate. What was not overcome was the requirement of proximity to a deep body of salt water, and difficulty in replacing the RO cells.
The requirement of proximity to a deep body of salt water was addressed in U.S. Pat. No. 4,125,463 to Chenoweth, which is incorporated by reference herein in its entirety. The Chenoweth patent describes placement of numerous reverse a osmosis (RO) units inside a well or other subterranean cavity for the purpose of producing fresh water from salt water. The Chenoweth invention is similar to that of Cole et al. in that salt water is passed from the surface to an RO unit below such that the hydrostatic head above the RO unit provides the pressure needed to operate the RO membrane, and adequate flushing is ensured by directing water from one end to the other through the housings of the RO units. The Chenoweth RO unit is merely subterranean (below ground) while the Cole et al. unit is submerged (below the surface of the water). This improvement, however, was still insufficient and was never commercialized. The threshold problem that still impedes commercialization in this area is the difficulty of servicing and replacing the RO units at great depths below the surface.
Thus, there exists a continuing need for apparatus and methods to cost effectively purify liquids through reverse osmosis.